User's Manual

Doc. No.
Rev.
Date
1.00
2011-12-08
Damm Cellular Systems A/S, Denmark
TETRAFLEX
®
V7.5 MANUAL - RX Diversity for BS41x
TetraFlex® 7.5 Manual 4-22
4.5 RX DIVERSITY FOR BS41X
The BS41x TETRA Base Stations are provided with a dual RX diversity scheme allowing the
connection of two RX antennas simultaneously to improve the RX sensitivity.
The Base Station can run with only one antenna connected. Connecting a second antenna
can however give a considerably improvement of the total BS sensitivity, especially under
fading conditions. It is therefore highly recommended to make use of the dual diversity
receiver feature.
The BS41x runs with a typical TX output power of about 10 W Tetra at the rack connector.
Compared with the typical output power of 1W of a handheld mobile station, a difference of
about 10dB in the link budget exists between down-link and up-link, if the same RX sensitivity
is present.
Considerably effort has been made in the BS41x to improve RX sensitivity to compensate for
the low output power of the handheld subscriber units.
The use of an RX Tower Mounted Amplifier removes the influence of the RX feeders,
effectively improving the link balance with the loss of the TX feeder, which could be 2 dB for a
typical system.
The use of a dual RX antenna diversity system (space diversity) can improve the sensitivity of
BS receiver by up to 7 dB and thereby almost compensate for the remaining difference in the
link budget.
This application note describes the implemented diversity scheme of the BS41x and gives
typical measured sensitivities and diversity improvements under various conditions.
4.5.1 Diversity implementation
The dual diversity system is implemented with two completely independent receive sections
from the RX antenna via the TMA, feeder and RX multi-coupler into the carrier unit. In the
carrier unit two independent RX sections exists with first and second IF and two Analog-to-
Digital converters connected directly to the DSP.
The DSP thereby receives two completely independent RX signals. First it tries to recover the
timing synchronization independently of the two signals and to make a signal quality
evaluation.
If the two signals are not very different in quality, they will be added and the combined signal
will be used to improve the sensitivity.
If the two signals are very different in quality the best signal will be selected and the other
signal will not be used. This is done when the addition of the bad signal to the good one
reduces the sensitivity.